The present invention generally relates to a level detector, and more particularly, to a level detector suitable for detection of whether or not the level of the contents, such as powder, grains or liquid, in a silo or hopper attains a predetermined one.
Generally, a level detector is provided to a container such as a tank, hopper or silo to determine the level of the contents, such as powder, grains or liquid, in the container. For example, two level detectors are provided at the upper and lower parts of a hopper, respectively, to detect whether or not the level of pellets of synthetic resin materials is at a predetermined one, in order to maintain a correct supply of pellets in the hopper.
Conventionally, the torque-motor type, capacitive type, phototube type, ultrasonic type and tuning-fork type level detector have been used for detection of the levels of such materials.
In the case of a torque-motor type level detector, a vane made to turn by a small torque motor is provided in a container. When the container is empty, the vane is not loaded. When contents are placed in the container, the vane is then loaded to actuate the limit switch. In this type of a level detector, the torque motor is made to turn continuously. Accordingly, the motor is easily wasted and the service life of the motor is shortened. Further, the vane projecting inwardly of the container disturbs the flow of the contents in the container, and the vane itself is likely to be damaged by the flow of the contents in the container.
In a capacitive-type level detector, the level of any object is detected according to the difference in capacitance between the tip and base of a projecting bar provided inside the container, which capacitance depends on the existence of such an object. However, when the detecting bar is contaminated, its sensitivity varies, causing the detector to fail. Further, when ambient conditions, for example, the humidity, change, the capacitance between said tip and base also changes. For this reason, it is difficult to present said detector at a desirable level. The detection bar projecting inside the container is pressed by the flow of the contents such as powder, grains or liquid, and is thus easily broken.
With a phototube type level detector, the level of the contents, such as of powder, grains or liquid, is detected by an arrangement whereby a light beam from a projector is interrupted by objects existing in the container. However, such level detection is impossible for any contents, which are transparent. Further, the beam receiver window of the phototube is easily dimmed by the contents, resulting in failures of said phototube type detector.
In the case of a tuning-fork type level detector, a projecting tuning fork activated by an exciter is provided inside the container. The level is detected based on the principle in which the oscillating state of the tuning fork varies depending on the varying volume of the contents in the container. However, there is a danger for the projecting tuning fork to be broken under the flow pressure.
Finally, the ultrasonic level detector detects the level of the contents in a container based on the reflection time of an ultrasonic wave between the top end of the container and the top surface of the contents contained therein. Disadvantageously such ultrasonic type detector is very expensive.